Coordinator stuck "Working" while only a background subagent runs — queued input not answered or shown as pending

### Describe the bug

Sharing something I've hit several times in the CLI (v1.0.70). Upfront: I don't really know how Copilot's orchestration works internally — the log analysis below was done by Copilot itself at my request. The parts I can vouch for first-hand are what I saw in the UI.

• The busy indicator showed "Working", and prompts I typed were pushed to a queue — not answered, and not shown as "(pending)".
• I tried Ctrl+X B (move task to background) and it said "No running sync task to move to background" — so the CLI itself says nothing sync is running, yet it's "Working" and queueing my input.
• In /tasks , the only thing running was one subagent, marked [background] .
• The coordinator became responsive only after I manually cancelled that background subagent in /tasks . Cancelling the sole [background] task is what released my queued prompt — which makes it look like a "background" task was actually blocking the main agent.

### Affected version

v1.0.70

### Steps to reproduce the behavior

_No response_

### Expected behavior

if the coordinator is free it should answer immediately; if busy, it should take my prompt as steering and show "(pending)" and act next turn. Actual: queued for minutes with nothing visibly happening except a background subagent.

### Additional context

From Copilot's log analysis (may be imperfect, not internal state): it found two windows where the coordinator went quiet for ~13–17 min with only a background subagent running. In the clearest one, the subagent-completion notification actually fired fine — so this isn't just a "dropped notification"; it looks more like the coordinator being blocked by / kept "Working" behind its own background subagent, and/or how typed input is classified while a subagent runs. It also ruled out sync-blocking (all read_agent calls were wait:false , tasks were mode:background ).

Bắt đầu bằng cách tái hiện hành vi trong CLI v1.0.70 đồng thời quan sát /tasks, chỉ báo Working, đầu vào được xếp hàng và Ctrl+X B. So sánh các phiên có một subagent [background] duy nhất và kiểm tra các cửa sổ log read_agent được mô tả trong báo cáo; được xem là hoàn tất khi coordinator phản hồi hoặc hiển thị rõ rằng đầu vào đang chờ xử lý mà không yêu cầu hủy thủ công.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.